The present invention relates, in general, to operational amplifiers and, more particularly, to a method and structure for enhancing the gain while reducing the input offset voltage of typical operational amplifiers.
Operational amplifiers generally comprise a first gain stage followed by an output stage the latter of which drives a load coupled to the output of the operational amplifier. The first gain stage includes at least an input differential amplifier stage the differential outputs of which drive a current mirror. The current mirror converts the differential outputs of the input differential amplifier to a single-ended output that drives the output stage.
The direct current (DC) gain of the aforesaid operational amplifier is typically limited by the voltage swing developed at the output of the first gain stage. Usually this voltage swing occurs at the output of the current mirror. This voltage swing causes a small current to be reflected back to the input differential amplifier due to the finite output impedance of the current mirror. This effect causes the input differential amplifier to correct for this imbalance which reduces the DC gain of the amplifier. CMOS transistor operational amplifiers are especially prone to this gain reduction due to the higher voltage swings necessary at the output of the first gain stage because of the low Gm of MOS transistors. Further, the output impedance of CMOS current mirrors is typically lower than for bipolar transistor current mirrors which will cause a greater current to be reflected to the input of the operational amplifier.
In view of the above, it would be advantageous to provide circuitry and a method for enhancing the DC gain and reducing the input offset voltage for operational amplifiers.